1. Field of the Invention
The present invention relates to a focus controlling method and apparatus in an electronic moving picture or still camera. The present invention further relates to using variable focus sampling periods and/or shutter speeds depending on a focus evaluation value, brightness of the image, and/or recording mode.
2. Description of the Related Art
Today, almost all video and electronic still cameras include an automatic focusing mechanism. One type of automatic focusing mechanism which may be applied in these cameras is a mountain climbing servo-type automatic focusing method. The focus control of this type of camera operates to drive a motor which is connected to a focusing lens so that a maximum focus evaluation value is obtained. The focus evaluation value plotted versus different lens positions has the general shape of a mountain with the peak of the mountain being the best focusing position. Typically, the focus motor is driven at a high speed when the sharpness is low and the motor is driven at slower speeds when the sharpness is high and the focus evaluation value approaches a maximum value.
One way of obtaining the focus evaluation value is to obtain the luminance or brightness component of a detected picture. The luminance or brightness component is passed through a high pass filter and then integrated. The sharper or more focused the picture, the more high frequencies will be present in the picture, indicating that the picture is more focused. However, as the focus evaluation value in this type of system is determined using the high frequency components of the luminance value, not only does the focus evaluation value depend on the extent to which the image is in focus but the focus evaluation value will also vary with the brightness of the image being detected.
In automatic focusing systems, noise of the focusing signal may be a problem. In particular, the flicker of artificial light sources causes the brightness of an image to change when the artificial light is being generated using an AC power source. For example, FIG. 1 illustrates the intensity of an artificial light source plotted over time. In FIG. 1, the changing brightness which has a frequency of 100 Hz is produced when an AC power supply is connected to the artificial light source. In the United States or other locations where the AC frequency is 60 Hz, the flicker frequency of artificial light is 120 Hz. The flickering occurs in both incandescent AC lighting and is especially troublesome for automatic focusing mechanisms in the presence of fluorescent light. Even though a flickering of 100 Hz or 120 Hz may present difficulty in an electronic focusing system, this flickering is not noticeable to the human eye.
In FIG. 1, if the image is sampled every 60 Hz as indicated by sampling periods S1, S2, S3, and S4, it can be seen that the artificial light intensity for these three periods is different. Therefore, if a focus evaluation value is based on or affected by the light intensity, it may be difficult to provide an adequate focusing mechanism when the artificial light intensity has a frequency of 100 Hz and the sampling frequency is 60 Hz.
Some regions of the world have a power supply frequency of 50 Hz whereas others, such as the United States have a frequency of 60 Hz. So that a single camera may be manufactured for use both in regions having 50 Hz power sources and 60 Hz power sources, a solution to this flicker problem is desired.
A known solution proposed to this problem is disclosed in Japanese Laid Open Patent Publication No. 5-236328 (1993), which is incorporated herein by reference. In this proposed solution, a sampling is performed with a frequency of 20 Hz. For example, the first sampling is at period S1 and subsequently at period S4, as illustrated in FIG. 1. As every third 60 Hz cycle ({fraction (1/60)} second period) has the same brightness when the light source has a 100 Hz flicker rate, sampling every 20 Hz (every third 60 Hz period) eliminates the flicker problem. Additionally, the above described solution also eliminates flicker problems when the flicker is 120 Hz because each 60 Hz sampling period will always have the same flicker. Typically, a frame of an image consists of an even field and an odd field which are assembled together to form a frame. In FIG. 1, an even field can be captured during sampling period Si, an odd field during S2, etc.
However, a problem with the above described solution is that when the focus evaluation value is sampled with a frequency of 20 Hz which is ⅓ of the vertical or field frequency (60 Hz) of the video signal, the time needed to focus the camera increases. If the focus sampling value is increased three-fold by changing the sampling frequency from 60 Hz to 20 Hz, the automatic focus evaluation takes almost three times the amount of time as is required when the sampling frequency is 60 Hz.
Accordingly, it is an object of the invention to provide a rapid automatic focusing system based on a luminance signal which operates in environments illuminated by artificial light sources powered by both 50 Hz and 60 Hz power sources. It is another object of this invention to provide this automatic focusing method in an amount of time which is quicker than conventional solutions.
It is a further object of the invention to provide an automatic focusing method which is usable in both moving picture and still picture recording modes which achieves a fast focusing value.
It is yet another object of this invention to provide a focusing mechanism usable in a still image mode which acquires a focussed image quickly by recording images as the best focussed point on the mountain climbing curve is passed.
These and other objects are accomplished by a novel focusing method and apparatus in which a focus evaluation result is determined based on a brightness of a detected image. Initially, the shutter speed is set at {fraction (1/60)} second, a field is captured every 60 Hz and the sampling frequency for focusing is set at 20 Hz. If a detected focus evaluation value is greater than a reference value, then the shutter speed is set to {fraction (1/100)} of a second and the sampling frequency is set to 60 Hz. As the frequency of the artificial light is 100 Hz when the power supply has a frequency of 50 Hz and the frequency of the flicker is 120 Hz when the frequency of the power supply is 60 Hz, flicker will be eliminated for both power supply frequencies because a shutter speed of 100 Hz will always result in an amount of light which is constant when the flicker frequency is 100 Hz. When the flicker frequency is 120 Hz, sampling every 60 Hz results in a constant amount of light. As a shutter speed of {fraction (1/100)} second captures less light than a shutter speed of {fraction (1/60)} second, the shutter speed can only be set to {fraction (1/100)} second when sufficient light exists.
When the camera is out of focus by a large amount, it is desirable to move the focusing lenses by large amounts in order to quickly bring the image into focus. However, when the focusing position becomes near, moving the focusing lens quickly will result in passing the focusing position and moving to a position where the image is again out of focus, passing over the peak of the mountain. Therefore, at the start of the focusing process, the sampling frequency is initially set to 60 Hz and the motor can be moved rapidly. As the focus point approaches, if the motor is moved slower and if the sampling frequency is set to 20 Hz which is a period of more than two fields, the focusing accuracy is improved. This is accomplished by obtaining the focus evaluation value after the driving of the motor is stopped.
Another feature of the invention allows for the camera to be used in both a still image recording mode and a moving image recording mode. Often, the shutter speed in the moving image recording mode varies when the object is being photographed whereas the shutter speed in the still recording mode is constant. The sampling frequency at the time of recording the still image is set to a different value than the sampling frequency of when moving images are recorded. In the still image recording mode, the sampling frequency can be equal to one field compared to a sampling frequency of three fields when in a moving image recording mode, thus shortening the focusing time for still images.
Another manner of decreasing the amount of time which is necessary to obtain a still image which is in focus is to record still images as the focus evaluation result is increasing and the best focusing position is being approached and passed. The focusing lens is continued to be moved until the best focusing position is passed, indicating that the focusing is becoming worse. It is necessary to focus past the best focus position in order to confirm that the best focus position was actually reached. Instead of backing the focusing lens to the position where the best focus occurs after the best focus position is passed, a still image is recorded for each of the focusing positions as the focusing position approaches the best focusing position. After the best focusing position is passed, a previously recorded image which corresponds to the best focusing position is used as the in-focus image and the other recorded images are discarded.